Empirical Estimates of Egg-to-Fry Survival in Yakima River Spring Chinook Salmon

Egg-to-fry survival is thought to be one of the major factors limiting freshwater production and recovery of Chinook salmon (Oncorhynchus tshwatscha) populations. Although there is a need for standardized methods to estimate survival over large temporal and spatial scales, most existing estimates consist of only a few samples and vary widely in methodology. A multi-year study was initiated in the fall of 2009 to evaluate the potential effects of spawning site and river reach on egg-to-fry survival. Three artificial redds were constructed at each of three sites, in nine contiguous reaches of the upper Yakima River. Artificial redds were stocked weekly with modified Whitlock-Vibert egg boxes containing bank-spawned eggs from one of nine male-female pairs. Stocking occurred over a three week period in September while natural spawning was occurring in the river. Measures of gravel composition, scour depth, and infiltration of fine sediment into the artificial redds were made at each site to evaluate potential specific factors affecting survival. Egg-to-fry survival averaged 72.5 percent and ranged from 0 to 99 percent in the first year. Survival was not significantly different between reaches but was detectably different between adult crosses. Levels of fine sediment in the artificial redds ranged between 0.04 and 21.3 percent (mean, 2.9) and were only weakly correlated with survival, explaining less than eight percent of the variation. Unusually mild winter conditions in the Yakima River in 2009-2010 resulted in little to no scour and few high flow events. Preliminary results suggest that adult fitness can play an important role in egg-to-fry survival in some years. Additional years of sampling are being conducted to evaluate inter-annual variation in survival during this life stage.